Fusion plates are used to immobilize and fuse adjacent spinal vertebrae following a discectomy (spinal disc removal) or for immobilizing the area surrounding a corpectomy (removal of an entire vertebral body). These procedures create a gap in the spine from the removed disc or vertebral body. The gap typically is closed by inserting a bone graft, usually from a cadaver. The adjacent vertebrae surrounding the discectomy or corpectomy site are then immobilized by attaching a fusion plate, usually on the anterior side of the spine, so that the vertebrae fuse to the bone graft, forming an entire fused section of the spine. Fusing of vertebrae to the bone graft requires that the vertebrae remain immobile.
The term “fusion” means achieving a rigid bone union. When metallic internal fixation devices were first introduced in the 1950s it soon became evident that the stress factors directed to this instrumentation by the human body turned out to be much greater than anticipated, leading to a quest for structurally stronger instrumentation rather than a quest for more functional and physiologic stabilizations.
Devices such as the Luqué ring allowed better fixation than its predecessors but the laminar wires needed to attach the ring to the lamina were unwieldy and, if the system was removed at a late date, the fixating wires, (which had an unfortunate tendency to erode through the adjacent dura mater), could create serious cerebro-spinal fluid leakage problems. Ring implants fell into disuse with spine surgeons when their attention became focused on pedicle screw fixation systems in the 1960s. Boucher, in 1959, has been credited as being the first to use pedicle screws but it was Roy-Camille, in the late 1970s, who was the first to use screws and hooks and connecting them with rods or plates. In 1983 Arthur Steffee patented the VSP spinal fixation system.
The early pedicle screw fixation systems utilized rather thick rods to support the screws. Not only were the rods difficult to bend at surgery but they tended to exert significant adverse stress on the screws and adjacent spinal segments. Complications involving nerve compression or injury associated with the placement of pedicle screws still occur in approximately 8-15% of cases. With heavy rods and large screws these problems tended to be increased. Initially, fractures of both pedicles and screws were fairly common.
An important advance in spinal fusion systems was the replacement of heavy plates with rods, particularly those allowing some degree of flexibility. Front and back (360° fusion) became possible, as well as anterior interbody femoral ring allografts placed at multiple levels. The front and back approach was the first type of fusion capable of producing consistently solid and rigid spine stabilization, even among patients who were smokers when multi-level rigid stabilization was considered desirable
Today, there are many different types of metal devices used to perform a lumbar fusion. These devices connect two or more vertebrae together, hold them in the correct position, and keep them from moving until they have a chance to grow together, or fuse. This is not an easy task. The vertebrae are small, so there is not much room to put in the screws, plates, or metal rods that surgeons typically use to connect bones. Another problem is that many nerves get in the way of putting screws into the vertebral body. Finally, there is a great deal of stress across the lumbar spine when standing upright or even sitting. Finding a metal device that is able to hold the bones together can be difficult.
Most of the current devices that surgeons favor use metal screws that are placed through the small tube of bone, called the pedicle, and into the vertebral body. These screws are attached to metal plates or metal rods that are bolted together in the back of the spine. This combination of hardware creates a solid “brace” that holds the vertebrae in place. These devices are intended to stop movement from occurring between the vertebrae that are being fused, allowing a solid fusion to occur. These metal devices give more stability to the fusion site and allow the patient to be out of bed much sooner.
The best applications of rigid pedicle screw and rod fixation systems have been in cases of post-traumatic spine instability, where the adjacent segments are basically normal. For patients with advanced scoliosis or deformity which can result in progressive pulmonary or neurologic impairment, or in the management of spinal trauma, pedicle screw systems can be life-saving.
Pedicle screws may be titanium or stainless steel construction. Screw design differs as well. For example there are:
Polyaxial screws, which allow the head to rotate and lock onto the rod at any angle, which is important for ease of insertion.
Monoaxial screws provide more rigidity and are important for deformity correction.
A tulip head allows for less rod bending since the head can angle to engage the rod.
Cannulated screws provide a central canal for placement over a K-wire during a percutaneous application.
Fenestrated screws are designed with multiple holes allowing for bony in-growth and/or influx of polymethylmethacrylate.
Vertebral stabilization systems containing both polyaxial pedicle screws and variations of vertebral hooks for effecting temporary vertebral fixation for spinal fusion procedures are currently available. These spinal systems are manufactured by large medical device companies such as Zimmer (ST360 Spinal Fixation System), Medtronic Sofamor Danek (CD Horizon Legacy System), DePuy Spine (of Johnson & Johnson) Fixation Systems, and Synthes (Click'X Spine System).
For at least the reasons discussed above, even rods and pedicle screws can be difficult to install, with small spaces, degenerate bone due to osteoporosis or cancer, and misaligned vertebrae.
It is therefore an object of the present invention to provide pedicle screws that are easier to install into the pedicles and to use to secure the spinal fixation system.
It is a further object of the invention to provide an improved method for securing a rod during spinal fusion surgery.